This application relates to battery packs of the type used for powering cordless power hand tools and the like, and to recharge circuits for such battery packs. In particular, the application relates to battery packs incorporating lead-acid battery cells.
Lead-acid battery cells of the type used for powering cordless power tools, such as thin metal film cells, do not tolerate deep discharges. Indeed, such cells are prone to permanent damage if left in a state of deep discharge for an extended period of time.
It is known to provide recharging devices for restoring or maintaining the charge on a lead-acid battery. Some such devices incorporate an auxiliary battery which typically is of a size and capacity comparable to the battery being recharged. Such devices are not suitable for use with battery packs of the type used for powering cordless power hand tools, since such battery packs must be of a relatively small size and weight. Furthermore, during normal operation of a cordless power tool, the battery pack terminal voltage can undergo significant momentary terminal voltage drops, depending on the load on the battery. For example, when the tool is turned on, its motor may draw approximately 20 amps for a fraction of a second until it comes up to operating speed. Furthermore, in the case of an impact tool, such as an impact wrench or the like, during the impact phase the motor may draw approximately 15 amps for about five seconds, a typical fastener installation period. Similar heavy loads will be drawn during the stall phase of a nut driver or screw driver tool. During such heavy current loads, the battery terminal voltage may dip below the voltage level which would be indicative of deep discharge, even though the battery is not, in fact, in deep discharge. Thus, a deep discharge recovery circuit which relies on battery terminal voltage to indicate a deep discharge condition must be able to distinguish from these normal, momentary voltage drops.